(adapted from WU2O)
Ignore most of the advice in the v3.5 user manual with regard to PureSignal, it is wrong. Leave HPF on TX enabled. You absolutely DO want to use an external coupler with an external amp. And play with "Relax Tolerance" and all of the other settings as much as you want (although the defaults are generally pretty good).

Short answer (setup):

a) Get a coupler. The Xtronic XDC-1 is a good one, but it's $130

b) Tell us how much power you are running and we can suggest if any external attenuation is required on the coupled output of the coupler. You do not want to exceed +13dBm feedback signal level at the rear panel connector of the radio under any conditions. Shooting for a +10dBm feedback signal level is a good idea.

d) Limit stitched receivers must be ON. Disable PureSignal must be OFF. EXT1 on TX must be ON. Auto-Attenuate (in the Linearity control panel) should be ON.

e) Microphone audio settings MUST be set to obtain an ALC reading of 0dB on peaks. PureSignal only kicks off correction measurements when it sees a 0dB ALC peak. Pushing up the audio settings to obtain peaks of +1dB on the ALC Comp meter is a good idea to ensure that this level will be reached reliably. 1dB of ALC action will be unnoticeable given the advanced ALC algorithm used in openHPSDR.

Short answer (operation):

a) Ensure DUP is on.

b) Set the scale of the lower end of the transmit spectral display to -60 and the top to 20 (in Setup > Display > Tx. This will give you the best view of your outgoing signal. Anything that is below -60 is only of academic interest. Ignore the waterfall during transmit, it does not scale properly and no mechanism is provided in openHPSDR to adjust it.

c) Tune up on frequency as normal with your antenna tuner and amplifier.

d) Open the PureSignal control panel.

e) Select "Autocalibrate".

f) Enable the Two-tone Generator and note that you are obtaining the correct output power and that PureSignal is operating and reducing IMD. The Correcting and Feedback Level indicators should be green and the signal should show good IMD (better than -50dBc) in the spectral display.

g) Disable the Two-tone Generator.

h) Operate the radio. On voice peaks you should see the Feedback Level indicator flash green. Monitor your spectral display during transmit and you should note the sharp transmit passband response with little if anything visible above -60 on the spectral display outside of the passband.

Steps f and g above are really optional and as you gain confidence with the operation of PureSignal you will find that they are really unnecessary most of the time.